代码如下:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en"> 
<head> 
<meta http-equiv="Content-Type" content="text/html;charset=UTF-8" /> 
<title>mask-layer</title> 
</head> 
<style type="text/css"> 
*{margin:0;padding: 0;} 
ul{margin:100px auto 0; width:200px;} 
img{border: 0 none;} 
li{background-color:#555;list-style: none; width: 146px; height: 220px;} 
a:hover{opacity: 0.6;} 
</style> 
<body> 
<ul> 
<li><a href=""><img src="http://h.hiphotos.baidu.com/baike/s%3D220/sign=700b0c26462309f7e36faa10420f0c39/64380cd7912397dd9e0581b65982b2b7d0a28707.jpg" alt="" /></a></li> 
</ul> 
</body> 
</html> 

原理:给li加上背景,li的大小就是图片的大小,在a:hover时候改变a的透明度